We propose to extend current knowledge on the mechanisms of action and resistance of the tetracycline antibiotics and to use the information so obtained to design, synthesize and test the properties of new derivatives of the tetracyclines (Tc). Several approaches will be used. New derivatives will be tested for their abilities to inhibit microbial growth and to overcome resistance. Several of the derivatives will be photolabile, and in radioactive form will be used in photoaffinity labeling experiments directed toward the identification and characterization of Tc binding sites on the E. coli ribosome. These experiments should enable us to more fully understand their inhibitory effect on protein synthesis. The same class of derivatives will be used to search for Tc binding sites within the membrane, thus opening the transport process to further study. Finally, radioactive Tc will be used to study the inducible nature of plasmid resistance.